Very low COD unbleached pulp

ABSTRACT

A pulp product that has a low COD is comprised of an unbleached pulp that is soaked and washed in alkaline water for a sufficient time so that the COD after soaking and washing is less than or equal to 2.0 kg/1000 kg of dry pulp. The pulp is produced by repetitively soaking and washing an unbleached pulp in alkaline water that is never allowed to drop below pH 7.0. The pulp is soaked and washed for a total of at least 220 minutes at an elevated temperature to produce the pulp product that has a low COD.

FIELD OF THE INVENTION

[0001] The present invention relates to cellulosic pulps, and moreparticularly, to unbleached cellulosic pulps having a low COD that areuseful in cementitious products.

BACKGROUND OF THE INVENTION

[0002] The internal structures of houses and other buildings arecommonly protected from environmental elements by exterior sidingmaterials. These siding materials are typically planks or panelscomposed of wood, concrete, brick, aluminum, stucco, wood composites, orfiber-cement composites. A common fiber-cement composite is fiber-cementsiding, which is generally composed of cement, silica sand, unbleachedwood pulp, and various additives. Fiber-cement siding offers severaladvantages over other types of siding materials, such as wood siding: itis weatherproof, relatively inexpensive to manufacture, fire-resistant,and invulnerable to rotting or insect damage.

[0003] Commercial fiber-reinforced cement siding planks or panels aremade using the Hatschek process. The Hatschek process was initiallydeveloped for the production of asbestos composites, but it is now usedfor the manufacture of non-asbestos, cellulose fiber reinforced cementcomposites. In this process, unbleached cellulose fibers are re-pulpedin warm water at an alkaline pH of 11 to 12.5; the re-pulped fibers arerefined and then mixed with cement, silica sand, and other additives toform a mixture. The fiber-cement mixture, is deposited on a felt bandsubstrate, vacuum dewatered, and cured to form a fiber reinforced cementmatrix in sheet form.

[0004] The curing of the cement matrix is hindered by the presence ofsugars or other organic materials. These materials retard the hydrationreaction of cement and thereby retard the setting or hardening of amortar or concrete. Cement setting is purposely retarded in ready-mixconcrete during long hours of transportation, for mitigation of stressdue to temperature (heat) when used in a large-sized concretestructures, and for decorated wash finishes. When these organicmaterials are measured, the manufacturers of fiber-cement siding haveobserved an inverse relationship between the amount of these materialsin an unbleached pulp and the strength properties of the final product.One means for measuring the amount of these materials is the chemicaloxygen demand (COD) test. When considering the detrimental effect ofthese materials on strength properties, it is apparent that there areneeds in the art for very low COD pulp. The present invention fulfillsthese needs and provides further related advantages.

SUMMARY OF THE INVENTION

[0005] The present invention provides a pulp product comprising anunbleached pulp that is soaked and washed only in alkaline water, andafter soaking and washing, it has a COD less than or equal to 2.0kg/1000 kg of dry pulp, and preferably less than or equal to 1.8 kg/1000kg. This pulp is produced by repetitively soaking unbleached pulp inalkaline water that is at an elevated temperature so that organics willdiffuse out of the fiber and into the water. Subsequently, therepetitively soaked and washed pulp is then preferably dewatered anddried.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The foregoing aspects and many of the attendant advantages ofthis invention will become more readily appreciated as the same becomebetter understood by reference to the following detailed description andthe accompanying drawings, wherein:

[0007]FIG. 1 illustrates the steps of the caustic washing process forthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0008] The present invention provides a low chemical oxygen demand (COD)pulp that is particularly useful for reinforcing fiber cement products.The low COD cellulosic pulp that is useful in the present invention ismost preferably an unbleached pulp from a Kraft (sulfate) pulpingprocess. Other low COD pulps and processes for making such pulps aredescribed in U.S. patent application Ser. No. 10/209,497, titled VERYLOW COD UNBLEACHED PULP and U.S. patent application Ser. No. 10/209,446,titled PROCESS FOR PRODUCING VERY LOW COD UNBLEACHED PULP, thedisclosures of which are herein expressly incorporated by reference intheir entirety. In addition, a PROCESS FOR MAKING LOW COD UNBLEACHEDPULP, U.S. patent application Ser. No. ______, Attorney Docket No. 25113filed herewith, is incorporated by reference in its entirety. However, awide variety of pulped cellulosic fibers can be used, which are derivedfrom wood and non-wood sources. Of all the pulp sources, wood pulp isthe most commonly employed because of its availability and price.

[0009] To obtain suitable wood pulp fibers, the Kraft (sulfate) pulpingprocess is the most preferred method. Using this process and consideringthe desired composite properties, the preferred wood fiber source islong-fibered coniferous wood species. Examples of these species includethe following: Southern pine, Douglas fir, spruce, hemlock, and Radiatapine. In addition to these wood fiber sources, other chemical pulps canbe used that include pulps made from short or long fibered wood speciesor recycled, wood pulp fibers. Other fibers that can be used includemini-chip pulp which is derived from sawdust, shavings, pin chips andother waste wood materials. The Weyerhaeuser Company of Federal Way,Washington, sells one such mini-chip pulp under the trademark TYEE.Short wood fibers, which are typically produced from hardwood speciessuch as eucalyptus, can also be used. The processes to produce thesewood pulp fibers are well known to those skilled in the art of pulpmanufacturing. These fibers are commercially available from a number ofcompanies, including the Weyerhaeuser Company. In contrast to wood pulpfiber sources, there are other natural cellulosic fiber sources whichinclude straw, flax, kenaf, hemp, or similar materials. Like wood-basedfibers, these non-wood fibers may also be pulped and subsequently usedin fiber cement-based composites. In addition, combinations or mixturesof cellulosic fibers can be used, such as mixtures of various wood pulpfibers, mixtures of wood pulp fibers and other natural cellulosic fibersor mixtures of other natural cellulosic fibers.

[0010] Referring to FIG. 1, unbleached pulp, preferably unbleached kraftpulp preferably with a Kappa number less than or equal to 30, is firstpassed through conventional brown stock washers 10 and a brown stockdecker 20. The unbleached pulp from the decker is at an alkalinecondition and must be maintained at an alkaline condition (pH greaterthan 7.0) while it is processed in accordance with the presentinvention. The unbleached pulp from the decker, which is at aconsistency of approximately 3-12%, may be transferred to diffusiontower 30 and held for a predetermined length of time and a predeterminedelevated temperature. This soaking step is preferably conducted for atleast 30 minutes at a temperature of at least 65° C., and morepreferably at approximately 70° C. or higher. Upon exiting the firstdiffusion tower 30, a sodium hydroxide solution may be mixed into thepulp slurry to the extent needed to maintain the pH above 7. Preferably,the consistency is lowered to about 4%. The resulting slurry may bedewatered in a press 40 to a constituency of about 30% then diluted to a10% consistency.

[0011] After the press 40, or the decker 20 if the tower 30 and press 40are not used, the pulp is placed in a high-density storage or brownstock storage tank 50 in which it can reside anywhere from 15 to 430minutes, but more preferably, for 120 minutes. The purpose of thehigh-density storage tank is to prevent flow variations from passing tothe downstream processes.

[0012] After the high-density storage tank 50, the pulp is thenintroduced into the first of a series 60 of diffusion towers andtreatments 70 at a consistency of approximately 3-12%. Hot water andsteam or only steam are added to the pulp in the second of the series 60of diffusion tanks to raise the temperature to a value that ispreferably equal to or greater than 55° C. The pulp is then repetitivelysoaked in the remaining series 60 of diffusion towers. After soaking ineach tower 60, the pulp is subjected to treatments 70. Preferably thepulp is soaked and washed or pressed multiple times, usually five (n=5)or six (n=6) times in a series of towers 60 and treatments 70, before itis forwarded to a storage tank 80. Each of the successive soaking anddiffusion steps may take a minimum of 15 minutes but may take up to 300or more minutes. These time periods include the treatment steps thatfollow each soaking step. This residence time is dependent on the sizeof the towers and level of pulp maintained in the towers.

[0013] It is preferred that the repetitive soaking steps in diffusiontowers 60 occur for between about 220 minutes to 420 minutes and ineither case at the elevated temperature of at least 55° C. It is alsopreferred that the temperatures in most of the towers in 60 be above 70°C. It is preferred that the total soaking time in diffusion tower 30,storage vessel 50, and diffusion towers 60 ranges from a minimum of 220minutes (0-30 minutes in diffusion tower 30, 0-30 minutes in vessel 50,220 minutes in diffusion towers 60) to a maximum of approximately 660minutes (0-60 minutes in diffusion tower 30, 660 minutes in diffusiontowers 60). Conventionally, however, the soak will occur forapproximately 330-420 minutes (0 minutes in diffusion tower 30, 330-420minutes in diffusion towers 60).

[0014] After the pulp has been repetitively soaked and treated, it maybe forwarded to the storage tank 80. The pulp may reside in storage tank80 for up to 430 minutes. However, generally it is preferred to have thepulp reside in storage tank 80 for about 100 minutes if the storage tank80 is employed. In this storage tank, the pulp remains at a consistencyof about 10%. Thereafter, the pulp is fed in a conventional manner to aconventional pulp machine 100 and dryer 110. After the pulp is driedinto sheets, it is cut to size, sent to storage 120, and ultimatelyshipped to a customer.

[0015] The unbleached and washed pulp produced by the process of thepresent invention has a COD of less than or equal to 2.0 kg per metricton (1,000 kg), and preferably less than 1.8 kg/1000 kg. This level ofCOD is well below that achieved in ordinary pulp mills and particularlyin kraft pulp mills producing unbleached pulp. Also, it is preferredthat the pulp produced by this process has a Kappa number less than 30and preferably less than or equal to 25±3.

[0016] A critical feature of the present invention is that the pulp mustbe maintained at alkaline conditions from the time it begins the initialsoaking in tower 30 until the pulp is dried. It is preferred that the pHbe maintained at or above 8.0, and more preferably between 9-11,throughout the entire process, from the soaking in the initial diffusiontower 30 through the repetitive soaks in towers 60 and treatments 70.Preferably in these steps, the pH is maintained in the range of from10.0 to 11.0. The pulp is then run through the pulp machine 100 and thedryer 110. In these steps, the pH may be reduced, e.g., to 8.0 to 8.5,but can be run lower. Under certain circumstances, it may be necessaryto add caustic solution (20% by weight aqueous sodium hydroxide) afterthe first diffusion tower 30 or after one of the towers 60. It has beenfound that the addition of caustic at the rate of at least 2 kg permetric ton, and more preferably 4 kg per metric ton, will be sufficientto maintain the alkalinity of the pulp above pH 8.0 throughout theprocess. It is generally only necessary to add caustic during theinitial portion of the run of the process. For example, at a throughputof about 25 to 40 tons per hour, the caustic needs to be added at therate of 4 kg per metric ton for the initial 24 to 48 hour period thatthe pulp is run through the initial tower 30 during a multi-day run (upto ten days). This will assure that the alkalinity will be maintainedabove pH 8.0 throughout the entire process, which includes the dryingstage.

[0017] Caustic may be added in line 32 after tower 30, if used.Alternatively or in addition, caustic may be added in line 34 after oneof the towers 60. In one embodiment of the present invention, tower 30and press 40 are not used and caustic is added after either the first orsecond tower 60.

[0018] In one preferred embodiment, the present invention is carried outin a bleach plant that is normally associated with a Kraft pulp mill andused to convert unbleached pulp to bleached pulp. A typical bleach planthas a series, typically five or more, of bleach reactors, in whichvarious bleaching agents such as chlorine dioxide are added. The bleachreactors may employ a variety of flow characteristics, such as downflow, up flow and combinations thereof. An oxygen delignificationreactor (diffusion tower 30) may precede the series of bleach reactors(diffusion towers 60). In accordance with the present invention, thebleach plant is converted to use with the present invention by firstcutting off the supply of bleaching agents to the bleach reactor, andthereafter, the supply of bleaching agents to the bleach reactors issequentially shut off as the pulp sequentially enters the bleachreactors.

[0019] In a preferred embodiment, caustic is introduced into the pulpvia line 34 after one of the towers at 4 kg per metric ton of pulp. Itis very important in this cycle that the temperature is maintained at orabove 55° C., and in most towers, preferably above 70° C. This isaccomplished by adding steam to the vessels, as necessary, to maintaintemperatures. After soaking in each bleach reactor vessel, the pulp isrun through conventional washing or pressing units (treatments 70) thatnormally follow each of the bleach reactors. In one embodiment, duringthe washing and pressing, the consistency may be increased to as high30% or as low as 4%. Preferably, fresh water is used to wash the pulp;however, white water or clean hot water from the associated pulp plantmay be used, if necessary.

[0020] After the pulp from the last bleach reaction vessel is washed, itis placed into the vessel that is normally used for bleached pulpstorage (vessel 80). Thereafter, it is diluted in successive steps to aconsistency of about 1.5% and fed into a pulp head box on the pulpmachine 100. The dewatered sheet produced on the pulp machine is thenrun through the pulp dryer 110, for example, a Flakt dryer. Afterdrying, the sheet is subsequently cut, stacked, stored, and, thereaftersent to shipping operations.

[0021] If desired, the pulp machine and pulp dryer can be eliminated andthe pulp can be introduced directly into a jet dryer. The jet dryerproduces a dried singulated fiber, particularly useful in somecementitious products. One suitable jet dryer for use in the presentinvention is a fluid energy Aljet Model Thermajet, X0870L, manufacturedby Fluid Energy Processing and Equipment Company. It is also possible tocompletely skip the drying stage and use the pulp in a never-driedstate.

[0022] The following examples are intended for illustrative purposesonly and are not intended in any way limit the invention.

EXAMPLE 1

[0023] Chemical oxygen demand (COD) is determined by the followingmethod. Pulp sheets produced in accordance with the preferred embodimentof the present invention just described are torn or cut into smallpieces (approximately 4 cm square). The small pieces are mixed and themoisture is measured in accordance with Tappi procedure T412 om-94.Forty grams of pulp, oven dried weight, are then carefully weighed. A2,000 ml solution of 0.01 N sodium hydroxide solution is prepared usingdistilled or deionized water and analytical grade sodium hydroxide.Thereafter, the pulp is placed in 2,000 ml of the 0.01 N sodiumhydroxide solution and placed in a disintegrater and disintegrated for15 minutes at 3,000 rpm in a British Pulp Evaluation Apparatus (orBritish disintegrater) described in Tappi 205 sp-95. The pulp slurry isthen vacuum filtered immediately after disintegration using a WhatmanNo. 3 filter paper. The filtration time is long enough so that amajority of the filtrate is passed through the filter. Two hundred andfifty ml of filtrate is separated for COD analysis. A sample ispreserved with 2.5 ml of 50% sulfuric acid. Thereafter, the COD of thefiltrate is measured using the titration method described in StandardMethods for the Examination of Water and Wastewater, 20th Edition, 1998,Method #5220C, “Closed Reflux, Titrimetric Method”. The COD content isthen calculated as kilograms per metric ton of pulp based on the ovendried weight of the pulp.

[0024] Pulp samples were randomly selected from several production runsthrough a converted bleach plant with five towers (n=5). Initially,unbleached Southern Pine pulp was processed in a brownstock washer 10and a brown stock decker 20 at a consistency of approximately 10% whenit exited from the decker, then introduced into brownstock storage.Then, the pulp was introduced into a converted bleach plant. After eachof the first two towers in the series, press washers provided treatmentto the pulp. After each of the last three towers, diffusion washersprovided treatment to the pulp. In each of the press washers, the pulpconsistency was changed from about 10% to about 30% then diluted back to10%.

[0025] The pulp was run at the rate of 29 tons per hour through theconverted bleach plant. Caustic was added after the first tower at arate of 4 kg per metric ton of pulp. Residence times in the fivesuccessive bleach vessels were 30, 75, 30, 45, and 150 minutesrespectively. Temperatures in the five successive bleach vessels were75° C., 75-80° C., 75-80° C., 75-80° C., and 75-80° C., respectively.Pulp samples were taken during production runs downstream from the Flaktpulp dryer and tested for COD as set forth above. Samples A, B, C, D, E,F, G, H and I were taken over a one-day production period. The resultsare set forth below. SAMPLE ID COD (kg/metric ton) A 1.5 B 1.65 C 1.5 D1.6 E 1.5 F 1.6 G 1.6 H 1.5 I 1.3

[0026] The pulp produced in accordance with the foregoing example issuitable for incorporation into cementious products, such as panels,planks, tilebacker boards and ceramic tile underlayment, exhibitingexcellent strength properties. The pulp may be incorporated into theseproducts at about 6-10% by weight pulp using the Hatschek process. Morepreferably, the pulp may be incorporated into these products at about 8%by weight.

EXAMPLE 2

[0027] Pulp samples were randomly selected from several production runsthrough the converted bleach plant with six towers. Initially, TYEE pulpwas processed in a brownstock washer 10 and a brown stock decker 20 at aconsistency of approximately 10-12%. Then, the pulp was introduced intoa high density storage 50. It was then diluted to about 4% with hotclean water. After the storage 50, the pulp was introduced into aconverted bleach plant. After each of the six towers in the series,washers provided treatment to the pulp. In each treatment, the pulpconsistency was changed from about 4% to about 10% then back to 4%.

[0028] The pulp was run at the rate of 15 tons per hour through theconverted bleach plant. Caustic was added after the second tower at arate of 4 kg per metric ton of pulp. Residence times in the fivesuccessive bleach vessels were 45, 15, 60, 120, 60 and 120 minutes,respectively. Temperatures in the six successive bleach vessels were 55°C., 65° C., 75-80° C., 75-80° C., 75-80° C., and 75-80° C.,respectively. Pulp samples were taken during production runs downstreamfrom the Flakt pulp dryer and tested for COD as set forth above. SamplesA, B and C were taken over a one-day production period. The results areset forth below. SAMPLE ID COD (kg/metric ton) A 1.47 B 1.37 C 1.42

[0029] The pulp produced in accordance with the foregoing example issuitable for incorporation into cementious products, such as panels,planks, tilebacker boards and ceramic tile underlayment, exhibitingexcellent strength properties. The pulp may be incorporated into theseproducts at about 6-10% by weight pulp using the Hatschek process. Morepreferably, the pulp may be incorporated into these products at about 8%by weight.

[0030] While the preferred embodiment of the invention has beenillustrated and described, it will be appreciated that various changescan be made therein without departing from the spirit and scope of theinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A pulp product,comprising unbleached pulp, wherein the pulp product has a COD that isless than or equal to 2.0 kg per 1000 kg dry pulp.
 2. The product ofclaim 1, wherein the pulp product has a COD that is less than 1.8 kg per1000 kg dry pulp
 3. The product of claim 1, wherein said pulp isrepetitively soaked and washed for at least 220 minutes at an elevatedtemperature.
 4. The product of claim 3, wherein said pulp is maintainedat a temperature of at least 70° C. for at least a portion of said 220minutes.
 5. The product of claim 1, wherein said pulp before washing hasa Kappa number of ≦30.
 6. The product of claim 1, wherein said pulpafter washing has a Kappa number of ≦30.
 7. The product of claim 6,wherein said pulp after washing has a Kappa number less than or equal to25±3.
 8. The product of claim 1, wherein the pulp is comprised ofSouthern Pine pulp.
 9. The product of claim 1, wherein the pulp iscomprised of Douglas Fir pulp.
 10. The product of claim 1, wherein thepulp is comprised of mini-chip pulp.
 11. The product of claim 1, whereinthe pulp is soaked and washed in water that is always at alkalineconditions.
 12. The product of claim 1, wherein the pulp is a Kraftpulp.
 13. A cementitious product containing the pulp of any one ofclaims 1 through 12.